Final Fantasy 13.
Ambitious abstract storyline, poorly presented. 
Ambitious take on the battle system (your mileage may vary)
Ambitious three game structure, later split apart in favor of direct sequels.
Ambitious environments, fully exploreable, scrapped
The game starts in a giant quasi-space station so big it could considered its own continent, where the stars in the night sky are in fact the urban lights of distant cities.
How can one make something so boring out of a setting so awesome?
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The Last Story:
I didn't dislike the faded colors of the artstyle and rather enjoyed the gameplay mechanics. The real waste came from the fact that while the game is supposed to take place on a small, second rate province at the edge of a much larger empire, Sakaguchi & co still had to twist the plot into a "save the world" story: they really should have kept the story focused on a smaller scale.
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Similarly, Final Fantasy XII could have been the best game in the main series. It had the most interesting and three-dimensional villains, an ambitious and sprawling world that comes so close to feeling really alive, some fantastic dialogue-writing, and a highly-political plot with a lot of potential.
Final Fantasy XII Is the best game in the main series, even if the final product did not fully materialize Matsuno's enormously ambitious vision.
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Ashe is pleasantly reserved and more interesting than Lightning
Ashe is one of the best character in the series, and I especially loved the way her personal dilemmas echoed the ideological conflict between Venat and its fellow Occurias.
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just threw off any momentum you had and there were a few key points where you had to grind to holy hell and back (I think roughly level 28-30ish? to get by that stupid dragon in the forest that would insta-poison/status effect you hardcore, that area was a choreeee to grind in).
It's actually quite easy to reach levels 50-60 in the southern part of the Golmore Jungle (near the entrance to the feywoods). The grinding spot is easy to access, and the leveling up reasonably fast, and quite frankly, the Elder Wrym never troubled me (in fact, I was kinda disapointed by how easy it fell the first time I fought it)
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Skyrim also could've been fucking AMAZING if they'd had bothered to have fun gameplay and hire actual writers. With some good writing and fun combat Skyrim could've seriously been one of the best RPG:s, well, ever.
Skyrim with fun gameplay and good writters exists: it's called Xenoblade.
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What this meant in practice was that you could NOT activate about 90% of the HtH's when you first encounter them! Consequently if you wanted to see these developments, you had to backtrack to do it once affinity had risen! It completely destroyed the illusion of the characters bonding throughout the journey and replaced it with the idea of them needlessly returning to previous locations just to chat.
Given the amount of backtracking done during sidequests, you'll come back to every heart to heart location several times through the game.
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On top on my head, I'll personally nominate as games that failed to live up to their potential Mass Effect 2, Assassin's Creed 3 and Fire Emblem Awakening.
No, not the third Mass Effect: I am firmly in the camp that consider that ME3 and especially its conclusion improved the trilogy. No: the game that messed things up the most for me was Mass Effect Two: they took a flawed RPG with Shooter elements and turned the gameplay into a bland corridor-shaped shooter (which to add insult to injury was glitchy as sin on the PS3: my game froze four times in a row while passing doorways in the Normandy) with a veneer of RPG elements. Fixing the submenus? Naaaaaah, we'll just eliminate virtually all customization option and put a bunch of purchasable upgrades instead.
Then add the fact that all the incoherences of the backstory started to become much much much more obvious: Earth went from being the Galaxy's Easter Island to a Rising superpower in only 35 years? the Citadel, with its precious little available habitable space is now populated at 20% by Humans? Homo Sapiens are supposed to have a lifespan of 150 years yet no one you meet is older than 60 and Anderson is unironically claiming to be in his "Twilight Years" at age 48? And of course, it's the game that goes the farthest into turning Shepard into a Heinleinesque quasi-fascistic power-fantasy: the invincible demi-goddess of war who's always right against everyone, especially these pesky elected officials and who win every battle by virtue of being that much stronger than everyone else, urgh...
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Assassin's Creed Three main problem (apart from being another glitchy PS3 port where I was beamed several sequences ahead, twice) came from the fact that it was often nearly impossible to success with a stealthy approach even in parts which were supposed to be stealthy, while taking a violent approach and killing everyone on site was virtually never penalyzed, which made the game at the same time infuriating and laughably easy, while destroying all feeling of immersion and killing all the interest I had in the series.
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As for Awakening, while its plot was obviously written as a duology, like most other Fire Emblems (Book One: a war between a formerly warmongering realm and its vengeful westerly neighbour leave the continent torn and weakened just as an invasion from oversea arrives; Book 2: united by necessity, the eastern continent drives the fighting back to its invader's doorsteps while the fell dragon's worshippers keep acting in the shadows), they tried to squish the story in a single game, which produced a main campaign way too short to properly show how high the stakes were, leaving me with the feeling that I had just finished the disappointing Reader Digest version of a much longer tale..
